1. Field of the Invention
This invention relates to curable organosiloxane compositions. More particularly, this invention relates to high consistency organosiloxane compositions curable by a platinum-catalyzed hydrosilylation reaction. The compositions contain a mixture of fluorinated and non-fluorinated polydiorganosiloxanes. Cured elastomers prepared using the present compositions exhibit unexpectedly high values of desireable physical properties such as tensile and tear strength, resiliency and elasticity.
2. Background Information
Curable compositions containing mixtures of fluorinated and non-fluorinated high consistency polydiorganosiloxanes are known. One of the more common repeating units that constitute at least a portion of the units present in fluorinated polydiorganosiloxanes is the methyl-3,3,3-trifluoropropylsiloxane unit. When compared with cured elastomers prepared from polydimethylsiloxanes, elastomers prepared from fluorinated polydiorganosiloxanes exhibit inferior physical properties such as tensile and tear strength and elasticity in combination with a higher solubility in polar organic liquids such as alcohols, ketones and esters. This reduction in properties is compensated for by a lower solubility and a higher resistance to degradation in the aliphatic hydrocarbons present in aviation and automotive fuels.
Because high consistency fluorinated and non-fluorinated polydiorganosiloxanes are inherently immiscible, elastomers prepared from silica-reinforced mixtures of these polymers may delaminate under stress, thereby substantially reducing the physical properties such as tensile And tear strength of the elastomer relative to the levels achievable using the individual polymers.
One approach to increasing the compatibility of fluorinated and non-fluorinated polydiorganosiloxanes is taught by Evans et al. in U.S. Pat. No. 4,960,811, which issued on Oct. 2, 1990. In accordance with the disclosure of this patent 100 parts of a modified fluorinated polydiorganosiloxane containing both vinyl and silanol groups is blended with from 1 to 10 parts of a polydimethylsiloxane gum containing from 1 to about 10 weight percent of vinyl radicals.
The modified fluorinated polydiorganosiloxanes described in the Evans et al. patent are reaction products of 100 parts of a cyclic fluorinated trisiloxane, and from 0.5 to 2 parts of a chain modifier mixture comprising specified amounts of 1) a vinyl-terminated diorganosiloxane copolymer containing up to 75 repeating units per molecule wherein from 50 to about 67 percent of the repeating diorganosiloxane are not fluorinated and 2) a silanol-terminated diorganosiloxane homopolymer containing from 3 to 6 fluorinated organosiloxane units per molecule.